The present invention relates generally to gripping tools and more particularly to a power-driven tweezers adapted for use on an automatic assembly machine for picking up a part and placing the part in a desired location on a device being assembled by the machine.
Automatic assembly machines are widely used in industry to assemble various kinds of products. One of the operations commonly performed by such machines is picking up a part and placing it in a desired location on a subassembly. A power-driven gripping tool carried by a movable arm or the like is used to pick up the part and place it in the desired spot. For example, a machine which assembles printed circuit boards uses a tweezers-like gripping tool to pick up an electronic component such as an integrated circuit chip from a parts bin and place the component on a printed circuit board.
A gripping tool which picks up an electronic component must be gentle enough not to damage the component. At the same time the tool must be able to grasp the component securely enough to carry it to the desired location on the circuit board, to orient it with respect to the wiring on the board, and to place it on the board without disturbing other nearby components which have not yet been soldered into place.
An example of the kinds of gripping tools which have been used to pick up electronic components and locate them on circuit boards is shown in FIGS. 1A and 1B. Oppositely-directed tweezer legs 11 and 13 and a tweezer leg 15 orthogonal to the legs 11 and 13 are carried by a shaft 17 (a fourth leg opposite the leg 15 is not shown). The legs 11 and 13 pivot about pins 19 and 21, respectively, through slots 23 and 25 defined in a casing 27 carried by the shaft 17. An upper extremity of the shaft 17 is attached to an assembly machine (not shown). A collar 29 and a pair of rollers 31 are carried by a tubular shaft 33 which slides up and down over the shaft 17, and another collar 35 and a pair of rollers 37 (one of which is not shown) are carried by a tubular shaft 39 which slides up and down over the shaft 33. Springs (not shown) urge the legs into a spread-apart configuration as depicted in FIG. 1A. A vacuum conduit 41 which slides up and down within the shaft 17 carries a chuck 43 having a plurality of small holes 45 in its lower surface.
To pick up a component, a downward force is exerted on the collar 29 as indicated by arrows 47, pressing the rollers 31 against upper extremities of the tweezer legs 11 and 13 as indicated by arrows 49 and urging the legs to pivot about their pivot points 19 and 21 whereby the lower extremities of the legs 11 and 13 converge on the component as indicated by arrows 51, tending to center the component between the legs 11 and 13. Then a downward force is exerted on the collar 35, causing the lower extremities of the other pair of legs to converge on the component and center it between those legs.
Next the chuck 43 is lowered into contact with the component as indicated by an arrow 53. A vacuum source (not shown) connected to an upper extremity of the conduit 41 generates a partial vacuum which acts through the openings 45 in the chuck 43 to hold the component. Then the tool is moved vertically and laterally as indicated by arrows 55 to carry the component to the desired location and then is rotated as indicated by an arrow 57 to correctly orient the component for placement. Finally the chuck 43 is lowered below the extremities of the tweezer legs to place the component on the circuit board without disturbing any nearby components.
Some components are not easily gripped by arcuately-moving gripping elements because the surfaces of such elements do not remain parallel as they converge. Gripping tools in which pivotal motions of levers are mechanically translated into parallel motions of gripping surfaces have been proposed; typical examples of such tools are models 7960, 8660 and 8460 parallel grippers made by PHD, Inc., Ft. Wayne, Ind., as described in PHD Technical Bulletin 922A.
As electronic components have become smaller and the density of these components on circuit boards has increased, the allowable margin for error in locating and orienting such parts on printed circuit boards has become very small. Contemporary surface mount integrated circuit chips are so tiny and have so many connection points that a placement error as little as a few thousandths of an inch can result in wrong connections and ultimate failure and rejection of the entire circuit board. Tools of the kinds described above for placing components on circuit boards have not been adequate in light of the extremely high placement accuracy which is now required, and there is a need for a tool which can grip a component such as a surface mount chip and place it in its designated location on a circuit board with sufficient accuracy to ensure correct interconnection with the wiring on a printed circuit board and without disturbing nearby unsecured components.